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[Support,Windows] Tolerate failure of CryptGenRandom

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Summary:
In `Unix/Process.inc`, we seed a random number generator from
`/dev/urandom` if possible, but if not, we're happy to fall back to
ordinary pseudorandom strategies, like the current time and PID.

The corresponding function on Windows calls `CryptGenRandom`, but it
//doesn't// have a fallback if that strategy fails. But `CryptGenRandom`
//can// fail, if a cryptography provider isn't properly initialized, or
occasionally (by our observation) simply intermittently.

If it's reasonable on Unix to implement traditional pseudorandom-number
seeding as a fallback, then it's surely reasonable to do the same on
Windows. So this patch adds a last-ditch use of ordinary rand(), using
much the same strategy as the Unix fallback code.

Reviewers: hans, sammccall

Reviewed By: hans

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D77553
This commit is contained in:
Simon Tatham 2020-04-07 09:18:09 +01:00
parent e102b60201
commit fbe3be181d
1 changed files with 31 additions and 11 deletions
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34
lib/Support/Windows/Process.inc
View File

@ -439,18 +439,38 @@ const char *Process::ResetColor() {
return 0;
}
unsigned Process::GetRandomNumber() {
HCRYPTPROV HCPC;
if (!::CryptAcquireContextW(&HCPC, NULL, NULL, PROV_RSA_FULL,
CRYPT_VERIFYCONTEXT))
ReportLastErrorFatal("Could not acquire a cryptographic context");
static unsigned GetRandomNumberSeed() {
// Generate a random number seed from the millisecond-resolution Windows
// system clock and the current process id.
FILETIME Time;
GetSystemTimeAsFileTime(&Time);
DWORD Pid = GetCurrentProcessId();
return hash_combine(Time.dwHighDateTime, Time.dwLowDateTime, Pid);
}
static unsigned GetPseudoRandomNumber() {
// Arrange to call srand once when this function is first used, and
// otherwise (if GetRandomNumber always succeeds in using
// CryptGenRandom) don't bother at all.
static int x = (static_cast<void>(::srand(GetRandomNumberSeed())), 0);
(void)x;
return ::rand();
}
unsigned Process::GetRandomNumber() {
// Try to use CryptGenRandom.
HCRYPTPROV HCPC;
if (::CryptAcquireContextW(&HCPC, NULL, NULL, PROV_RSA_FULL,
CRYPT_VERIFYCONTEXT)) {
ScopedCryptContext CryptoProvider(HCPC);
unsigned Ret;
if (!::CryptGenRandom(CryptoProvider, sizeof(Ret),
if (::CryptGenRandom(CryptoProvider, sizeof(Ret),
reinterpret_cast<BYTE *>(&Ret)))
ReportLastErrorFatal("Could not generate a random number");
return Ret;
}
// If that fails, fall back to pseudo-random numbers.
return GetPseudoRandomNumber();
}
typedef NTSTATUS(WINAPI* RtlGetVersionPtr)(PRTL_OSVERSIONINFOW);